The invention relates generally to ceramic honeycomb articles, and more particularly to systems and methods for purifying diesel exhaust gases including such honeycomb articles. More specifically, the invention relates to flow-through honeycomb substrates and to methods and systems including combinations of flow-through substrates and wall-flow particulate filters.
Combustion of diesel fuel produces particulates including soot. These particulates are in addition to traditional fuel combustion emissions such as carbon monoxide, hydrocarbons, and nitrogen oxides. Wall-flow particulate filters are often used in diesel engine systems to remove particulates from exhaust gas. These wall-flow particulate filters are typically made of a honeycomb substrate with parallel flow channels and internal porous walls. The flow channels are plugged, usually in a checkerboard pattern, so that exhaust gas, once inside the honeycomb substrate, is forced to pass through the internal porous walls, whereby the porous walls retain a portion of the particulates in the exhaust gas.
Wall-flow particulate filters have been found to be effective in removing particulates from exhaust gas. However, pressure drop across the honeycomb filter increases as the particulates trapped in the porous walls increase. In a diesel-powered vehicle, this increasing pressure drop results in a gradual rise in back pressure against the diesel engine. When the pressure drop across the honeycomb substrate reaches a certain level, the wall-flow particulate filter may be thermally regenerated in-situ. Thermal regeneration involves subjecting the wall-flow particulate filter to a temperature sufficient to fully combust soot.
During thermal regeneration, excessive temperature spikes at various points in the honeycomb filter can occur due to poor control of the thermal regeneration. These excessive temperature spikes may produce thermal stress in the honeycomb filter. If the thermal stress exceeds the internal mechanical strength, the wall-flow particulate filter may crack, which may, in some cases, degrade performance. Therefore, means of better controlling regeneration temperatures in the wall-flow particulate filter during thermal regeneration is desirable.